Dosing procedures for
plant nutrients & additives

For nutrients and additives,
proper dosing procedures need to be applied in order to obtain maximum
performance.

Phase of growth (seedling, vegetative or flowering)

Is the plant in the seedling, vegetative or
flowering phase? This is important for determining which nutrient and
additives are required:

- Seedling phase: This phase begins when
seeds or clones are first “planted”. It continues until a significant root
system has developed and at least a few leaves have formed. Seedlings are
grown in a dedicated propagation unit then transferred into a bigger system
for the vegetative phase.

- Vegetative (or “growth”) phase:
Further foliage and root development occurs during this phase. Strong
vegetative growth in the plant is needed in this phase in order for there to
be a successful flowering phase.

- Flowering (“bloom” or “fruiting”) phase:
The flowering phase essentially begins when floral buds start to form and
continues until harvest.

Dosing procedure

Step 1. Fill
nutrient reservoir with water: Add the majority of water before the
nutrients and additives.

For recirculating systems, allocate at least 10
litres (2.5 gal) for each large plant (e.g. tomato), or at least 2 litres (½
Gal) for each small plant (e.g. lettuce). These volumes will help minimize
changes in nutrient concentration (EC) and pH. In hot weather, insufficient
nutrient volume could result in EC getting too high, resulting in leaf burn.
Larger nutrient volumes will also reduce how frequently top-up water is
needed.

Step 2. Add nutrient and additives:

- Choose the necessary nutrients and additives
based upon which phase of growth the plants are in - see section above.

- The dose rate depends upon factors such as
the type of medium, plant type and the phase of growth. Refer to the
manufacturers label for this information.

- Always add nutrients and additives
separately. Never premix them. For example, with 2 and 3-part nutrients, the
“parts” are kept separate for good reason. When these parts are mixed
together in a concentrated form (or in too little water), a white
precipitate will be produced (Fig 9.1).

- Stir well after adding each nutrient and
additive.

Step 3. Immediately check pH: Once all
nutrients and additives have been added, immediately check pH and adjust if
necessary. Maintain pH between 5.0 and 6.5 using pH Up or Down. For
recirculating systems, check pH daily.

Step 5. Maintaining nutrient
concentration (For recirculating hydroponic systems only): As plants grow
they simultaneously remove both water and nutrients from the solution.
Ensure the water level is kept relatively constant. When this is done, the
concentration (EC*) will be relatively predictable. It will move up or down
depending upon the size and growth rate of plants and the salinity of the
top-up water. *See
Table 11.30.

Step 6: Further notes:

- High pH additives: The best dosing
technique to adopt with additives that increase pH significantly (e.g.
silica and PK additives) is to add them to the water and adjust the pH down
to ~6 prior to adding the nutrient.

If the nutrient has already been added,
pre-dilute the additive in a separate volume of plain water. After this
solution is added to the nutrient solution, immediately lower the pH to
below 6.5. A white ‘cloudy’ precipitate (calcium sulfate) may form where the
pre-diluted additive initially merges with the nutrient solution. However,
because the initial particle size of the precipitate is small, it will
rapidly re-dissolve if the pH is immediately re-adjusted (Fig 9.2a).

For those additives that lower pH, it is
satisfactory to add them to the nutrient solution without pre-dilution.

- 2 & 3-part nutrients: Accurately
measure out the dose. In the case of a 2-part, 'under' dosing part 'B' for
example, could cause a deficiency in over half the nutrients required (i.e.
P, K, S and all of the trace elements - except iron).